Potassium hydroxide (KOH), commonly known as caustic potash, is an inorganic compound with a wide range of industrial and chemical applications. Understanding its mechanism of action is crucial for its effective and safe use. Here, we delve into the chemical properties, reactions, and functions of potassium hydroxide.
Potassium hydroxide is a strong base and is highly soluble in water, leading to the formation of a strongly alkaline solution. One of the critical aspects of KOH is its ability to dissociate completely in water. This dissociation process involves the separation of KOH into potassium ions (K⁺) and hydroxide ions (OH⁻):
KOH (s) → K⁺ (aq) + OH⁻ (aq)
The hydroxide ions (OH⁻) generated in this reaction are responsible for the high alkalinity of the solution. These ions can readily interact with various substances, making potassium hydroxide an essential reagent in many chemical processes.
One of the primary mechanisms of potassium hydroxide is its role in neutralization reactions. When KOH is mixed with acids, it reacts to form water and a corresponding potassium salt. For example, when potassium hydroxide is combined with hydrochloric acid (HCl), the reaction produces potassium chloride (KCl) and water (H₂O):
KOH (aq) + HCl (aq) → KCl (aq) + H₂O (l)
This neutralization process is vital in various industrial applications, including the manufacture of soaps, detergents, and other cleaning agents. In these applications, KOH helps to saponify fats and oils, converting them into soap and glycerol.
Another crucial mechanism of potassium hydroxide is its ability to act as a strong nucleophile in organic chemistry. The hydroxide ion (OH⁻) is highly reactive and can attack electrophilic carbon atoms in organic compounds, leading to substitution and elimination reactions. For instance, potassium hydroxide can dehydrohalogenate alkyl halides, resulting in the formation of alkenes:
R-CH₂-CH₂-X + KOH (ethanol) → R-CH=CH₂ + KX + H₂O
In this reaction, the halide (X) is removed from the alkyl halide (R-CH₂-CH₂-X) by the hydroxide ion, leading to the formation of an alkene (R-CH=CH₂).
Furthermore, potassium hydroxide is widely used in the production of biodiesel. In the transesterification process, KOH serves as a catalyst, enabling the conversion of triglycerides (fats and oils) into biodiesel (methyl or ethyl esters) and glycerol. The basic conditions provided by KOH facilitate the breakdown of triglycerides and their subsequent reaction with methanol or ethanol.
KOH also plays a significant role in the field of analytical chemistry, particularly in titration processes. Its strong basicity makes it an excellent titrant for determining the concentration of acidic solutions. The end point of such titrations is typically indicated by a pH indicator that changes color at a specific pH level, allowing for precise measurements.
Moreover, potassium hydroxide is utilized in the preparation of potassium salts, which are essential in various chemical syntheses and industrial processes. For example, potassium carbonate (K₂CO₃) and potassium phosphate (K₃PO₄) are derived from KOH and are used in the manufacture of glass, fertilizers, and food additives.
In summary, the mechanism of potassium hydroxide involves its strong basicity, complete dissociation in water, and ability to neutralize acids, act as a nucleophile in organic reactions, catalyze biodiesel production, and serve as a titrant in analytical chemistry. Its diverse applications, ranging from soap production to biodiesel synthesis and chemical analysis, highlight its significance in both industrial and laboratory settings. Understanding these mechanisms ensures the efficient and safe utilization of potassium hydroxide in various processes.
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Inorganic compound (KOH)
Potassium hydroxide
Names
IUPAC name
Potassium hydroxide
Other names
- Caustic potash
- Lye
- Potash lye
- Potassia
- Potassium hydrate
- KOH
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard
100.013.802
EC Number
E number
E525 (acidity regulators, ...)
PubChem CID
RTECS number
UNII
UN number
CompTox Dashboard
(EPA)
- InChI=1S/K.H2O/h;1H2/q+1;/p-1 YKey: KWYUFKZDYYNOTN-UHFFFAOYSA-M Y
- InChI=1/K.H2O/h;1H2/q+1;/p-1Key: KWYUFKZDYYNOTN-REWHXWOFAT
Properties
KOH
Molar mass
56.105 g·mol−1
Appearance
white solid, deliquescent
Odor
odorless
Density
2.044 g/cm3 (20 °C)
[1
] 2.12 g/cm3 (25 °C)
[2
]
Melting point
410
[3
][4
] °C (770 °F; 683 K)
Boiling point
1,327 °C (2,421 °F; 1,600 K)
85 g/100 mL (−23.2 °C)
97 g/100 mL (0 °C)
121 g/100 mL (25 °C)
138.3 g/100 mL (50 °C)
162.9 g/100 mL (100 °C)
[1
][5
]
Solubility
soluble in alcohol, glycerol
insoluble in ether, liquid ammonia
Solubility in methanol
55 g/100 g (28 °C)
[2
]
Solubility in isopropanol
~14 g / 100 g (28 °C)
Acidity (p
Ka)
14.7
[6
]
Magnetic susceptibility (χ)
−22.0·10−6 cm3/mol
Refractive index (
nD)
1.409 (20 °C)
Thermochemistry
Heat capacity
(C)
65.87 J/mol·K
[2
]
Std molar
entropy
(S⦵298)
79.32 J/mol·K
[2
][7
]
Std enthalpy of
formation
(ΔfH⦵298)
−425.8 kJ/mol
[2
][7
]
Gibbs free energy
(ΔfG⦵)
−380.2 kJ/mol
[2
]
Hazards
GHS labelling:
[8
]
Danger
H290, H302, H314
[8
]
P280, P305+P351+P338, P310
[8
]
NFPA 704 (fire diamond)
Flash point
nonflammable
Lethal dose or concentration (LD, LC):
LD50 (median dose)
273 mg/kg (oral, rat)
[10
]
NIOSH (US health exposure limits):
PEL (Permissible)
none
[9
]
REL (Recommended)
C 2 mg/m3
[9
]
IDLH (Immediate danger)
N.D.
[9
]
Safety data sheet (SDS)
ICSC
Related compounds
Other anions
Potassium hydrosulfide
Potassium amide
Other cations
Lithium hydroxide
Sodium hydroxide
Rubidium hydroxide
Caesium hydroxide
Related compounds
Potassium oxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N (what is
YN ?)
Chemical compound
Potassium hydroxide is an inorganic compound with the formula KOH, and is commonly called caustic potash.
Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which utilize its caustic nature and its reactivity toward acids. An estimated 700,000 to 800,000 tonnes were produced in . KOH is noteworthy as the precursor to most soft and liquid soaps, as well as numerous potassium-containing chemicals. It is a white solid that is dangerously corrosive.[11]
Properties and structure
[edit]
KOH exhibits high thermal stability. Because of this high stability and relatively low melting point, it is often melt-cast as pellets or rods, forms that have low surface area and convenient handling properties. These pellets become tacky in air because KOH is hygroscopic. Most commercial samples are ca. 90% pure, the remainder being water and carbonates.[11] Its dissolution in water is strongly exothermic. Concentrated aqueous solutions are sometimes called potassium lyes. Even at high temperatures, solid KOH does not dehydrate readily.[12]
Structure
[edit]
At higher temperatures, solid KOH crystallizes in the NaCl crystal structure. The OH− group is either rapidly or randomly disordered so that it is effectively a spherical anion of radius 1.53 Å (between Cl− and F− in size). At room temperature, the OH− groups are ordered and the environment about the K+ centers is distorted, with K+−OH− distances ranging from 2.69 to 3.15 Å, depending on the orientation of the OH group. KOH forms a series of crystalline hydrates, namely the monohydrate KOH · H2O, the dihydrate KOH · 2H2O and the tetrahydrate KOH · 4H2O.[13]
Reactions
[edit]
Solubility and desiccating properties
[edit]
About 112 g of KOH dissolve in 100 mL water at room temperature, which contrasts with 100 g/100 mL for NaOH.[14] Thus on a molar basis, KOH is slightly more soluble than NaOH. Lower molecular-weight alcohols such as methanol, ethanol, and propanols are also excellent solvents. They participate in an acid-base equilibrium. In the case of methanol the potassium methoxide (methylate) forms:[15]
- KOH + CH3OH → CH3OK + H2O
Because of its high affinity for water, KOH serves as a desiccant in the laboratory. It is often used to dry basic solvents, especially amines and pyridines.
As a nucleophile in organic chemistry
[edit]
KOH, like NaOH, serves as a source of OH−, a highly nucleophilic anion that attacks polar bonds in both inorganic and organic materials. Aqueous KOH saponifies esters:
- KOH + RCOOR' → RCOOK + R'OH
When R is a long chain, the product is called a potassium soap. This reaction is manifested by the "greasy" feel that KOH gives when touched; fats on the skin are rapidly converted to soap and glycerol.
Molten KOH is used to displace halides and other leaving groups. The reaction is especially useful for aromatic reagents to give the corresponding phenols.[16]
Reactions with inorganic compounds
[edit]
Complementary to its reactivity toward acids, KOH attacks oxides. Thus, SiO2 is attacked by KOH to give soluble potassium silicates. KOH reacts with carbon dioxide to give potassium bicarbonate:
- KOH + CO2 → KHCO3
Manufacture
[edit]
Historically, KOH was made by adding potassium carbonate to a strong solution of calcium hydroxide (slaked lime). The salt metathesis reaction results in precipitation of solid calcium carbonate, leaving potassium hydroxide in solution:
- Ca(OH)2 + K2CO3 → CaCO3 + 2 KOH
Filtering off the precipitated calcium carbonate and boiling down the solution gives potassium hydroxide ("calcinated or caustic potash"). This method of producing potassium hydroxide remained dominant until the late 19th century, when it was largely replaced by the current method of electrolysis of potassium chloride solutions.[11] The method is analogous to the manufacture of sodium hydroxide (see chloralkali process):
- 2 KCl + 2 H2O → 2 KOH + Cl2 + H2
Hydrogen gas forms as a byproduct on the cathode; concurrently, an anodic oxidation of the chloride ion takes place, forming chlorine gas as a byproduct. Separation of the anodic and cathodic spaces in the electrolysis cell is essential for this process.[17]
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Uses
[edit]
Photoresist Stripper Market Growth Outlook, 2025 to 2035Nitrile vs. Latex Gloves: Which Is Best for Your BusinessKOH and NaOH can be used interchangeably for a number of applications, although in industry, NaOH is preferred because of its lower cost.
Catalyst for hydrothermal gasification process
[edit]
In industry, KOH is a good catalyst for hydrothermal gasification process. In this process, it is used to improve the yield of gas and amount of hydrogen in process. For example, production of coke from coal often produces much coking wastewater. In order to degrade it, supercritical water is used to convert it to the syngas containing carbon monoxide, carbon dioxide, hydrogen and methane. Using pressure swing adsorption, various gases could be separated, and then power-to-gas technology is used to convert them to fuel.[18] On the other hand, the hydrothermal gasification process could degrade other waste such as sewage sludge and waste from food factories.
Precursor to other potassium compounds
[edit]
Many potassium salts are prepared by neutralization reactions involving KOH. The potassium salts of carbonate, cyanide, permanganate, phosphate, and various silicates are prepared by treating either the oxides or the acids with KOH.[11] The high solubility of potassium phosphate is desirable in fertilizers.
Manufacture of soft soaps
[edit]
The saponification of fats with KOH is used to prepare the corresponding "potassium soaps", which are softer than the more common sodium hydroxide-derived soaps. Because of their softness and greater solubility, potassium soaps require less water to liquefy, and can thus contain more cleaning agent than liquefied sodium soaps.[19]
As an electrolyte
[edit]
Aqueous potassium hydroxide is employed as the electrolyte in alkaline batteries based on nickel-cadmium, nickel-hydrogen, and manganese dioxide-zinc. Potassium hydroxide is preferred over sodium hydroxide because its solutions are more conductive.[20] The nickel–metal hydride batteries in the Toyota Prius use a mixture of potassium hydroxide and sodium hydroxide.[21] Nickel–iron batteries also use potassium hydroxide electrolyte.
Food industry
[edit]
In food products, potassium hydroxide acts as a food thickener, pH control agent and food stabilizer. The FDA considers it generally safe as a direct food ingredient when used in accordance with Good Manufacturing Practices.[22] It is known in the E number system as E525.
Niche applications
[edit]
Like sodium hydroxide, potassium hydroxide attracts numerous specialized applications, virtually all of which rely on its properties as a strong chemical base with its consequent ability to degrade many materials. For example, in a process commonly referred to as "chemical cremation" or "resomation", potassium hydroxide hastens the decomposition of soft tissues, both animal and human, to leave behind only the bones and other hard tissues.[23] Entomologists wishing to study the fine structure of insect anatomy may use a 10% aqueous solution of KOH to apply this process.[24]
In chemical synthesis, the choice between the use of KOH and the use of NaOH is guided by the solubility or keeping quality of the resulting salt.
The corrosive properties of potassium hydroxide make it a useful ingredient in agents and preparations that clean and disinfect surfaces and materials that can themselves resist corrosion by KOH.[17]
KOH is also used for semiconductor chip fabrication (for example anisotropic wet etching).
Potassium hydroxide is often the main active ingredient in chemical "cuticle removers" used in manicure treatments.
Because aggressive bases like KOH damage the cuticle of the hair shaft, potassium hydroxide is used to chemically assist the removal of hair from animal hides. The hides are soaked for several hours in a solution of KOH and water to prepare them for the unhairing stage of the tanning process. This same effect is also used to weaken human hair in preparation for shaving. Preshave products and some shave creams contain potassium hydroxide to force open the hair cuticle and to act as a hygroscopic agent to attract and force water into the hair shaft, causing further damage to the hair. In this weakened state, the hair is more easily cut by a razor blade.
Potassium hydroxide is used to identify some species of fungi. A 3–5% aqueous solution of KOH is applied to the flesh of a mushroom and the researcher notes whether or not the color of the flesh changes. Certain species of gilled mushrooms, boletes, polypores, and lichens[25] are identifiable based on this color-change reaction.[26]
Safety
[edit]
Potassium hydroxide is a caustic alkali and its solutions range from irritating to skin and other tissue in low concentrations, to highly corrosive in high concentrations. Eyes are particularly vulnerable, and dust or mist is severely irritating to lungs and can cause pulmonary edema.[27] Safety considerations are similar to those of sodium hydroxide.
The caustic effects arise from being highly alkaline, but if potassium hydroxide is neutralised with a non-toxic acid then it becomes a non-toxic potassium salt. It is approved as a food additive under the code E525.
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See also
[edit]
- Potash
- Soda lime
- Saltwater soap – sailors' soap
References
[edit]
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